A transient red colour: the aqueous chemistry between iron (III) ions and sulfur oxoanions

During this reaction a transient red colour is observed which is also seen when aqueous solutions of sulphur based salts are added to aqueous iron(III) ions

Equipment

Chemicals

20 cm³ of 1 mol dm^–3 solutions of each of: 

• Sodium sulphide (Na₂S)
• Sodium thiocyanate (NaSCN)
• Sodium sulphate (Na₂SO₄)
• Sodium sulphite (Na₂SO₃) 
• Sodium thiosulphate (Na₂S₂O₃)
• Sodium metabisulphite (Na₂S₂O₅)
• Sodium dithionite (Na₂S₂O₄)
• Sodium pyrosulphate (Na₂S₂O₇)
• Sodium tetrathionate (Na₂S₂O₆) 
• Sodium dithionate (Na₂S₄O₆)
• Sodium persulphate (Na₂S₂O₈)
• Iron(III) chloride (FeCl₃)

Apparatus

• Test tubes
• Test tube racks
• Spatulas
• Dropping pipettes
• Aqueous 0.5% (w/v) iron(III) chloride solution
• Safety glasses

Health, safety and technical notes

• Read our standard health and safety guidance here.
• Wear eye protection.
• The test tube samples can be washed to waste with plenty of running water. Larger quantities of most should be kept for disposal.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• Hazards are given for the solids – where no mention is made of the solution, the classification is the same.
• Sodium sulphide (Na₂S) is corrosive to skin and eyes, harmful if swallowed, toxic in contact with skin, contact with acids liberates toxic gas. Toxic to aquatic organisms.
• Sodium thiocyanate (NaSCN) is harmful if swallowed, inhaled or in contact with the skin; Contact with acids liberates toxic gas. (1 mol dm^–3 solution is of low hazard) 
• Sodium sulfate (Na₂SO₄) is of no significant hazard.
• Sodium sulfite (Na₂SO₃) is harmful if swallowed, causes serious eye damage, contact with acids liberates toxic gas.
• Sodium thiosulfate (Na₂S₂O₃) is of no significant hazard.
• Sodium metabisulfite (Na₂S₂O₅) is harmful if swallowed, causes serious eye damage, contact with acids liberates toxic gas. (1 mol dm^–3 solution causes serious eye damage and in contact with acids liberates toxic gas).
• Sodium dithionite (Na₂S₂O₄) is a self-heating solid – it may catch fire. Harmful if swallowed. (1 mol dm^–3 solution is of low hazard).
• Sodium pyrosulfate (Na₂S₂O₇) is of no significant hazard. 
• Sodium tetrathionate (Na₂S₂O₆) is a skin, eye and respiratory irritant.
• Sodium dithionate (Na₂S₄O₆) is of no significant hazard. 
• Sodium persulfate (Na₂S₂O₈) is a skin/respiratory sensitiser, skin/eye/respiratory irritant, harmful if swallowed. (1 mol dm^–3 solution is a skin/respiratory sensitiser and a skin/eye irritant).
• See CLEAPSS Hazcard HC088 for more information on sodium.
• Iron(III) chloride (FeCl₃) is corrosive to skin/eyes, harmful if swallowed, Hazardous to the aquatic environment. 

Commentary 

Students should be encouraged to draw out the structures of the sulphur-based anions and consider how the anions might interact with the Fe³⁺ ion.

If 3 cm³ of the sulphur anion solution is mixed with the same volume of iron(III) solution, then the solutions in bold type will give a transient red colour: 

• Sodium sulfide (Na₂S)
• Sodium thiocyanate (NaSCN)
• Sodium sulfate (Na₂SO₄)
• Sodium sulfite (Na₂SO₃)
• Sodium thiosulfate (Na₂S₂O₃)
• Sodium metabisulfite (Na₂S₂O₅)
• Sodium dithionite (Na₂S₂O₄)
• Sodium pyrosulfate (Na₂S₂O₇)
• Sodium tetrathionate (Na₂S₂O₆)
• Sodium dithionate (Na₂S₄O₆)
• Sodium persulfate (Na₂S₂O₈)

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.